In the gas-phase process for production of polyolefins, a gaseous alkene (e.g., ethylene), hydrogen, co-monomer and/or other raw materials may be converted to solid polyolefin (e.g., polyethylene) product. Generally, gas-phase reactors may include a fluidized-bed reactor, a compressor, and a cooler (heat exchanger). The reaction may be maintained in a two-phase fluidized bed of granular polyolefin and gaseous reactants by the fluidizing gas which is passed through a distributor plate near the bottom of the reactor vessel. Catalyst may be added to the fluidized bed. Heat of reaction may be transferred to the circulating gas stream. This gas stream may be compressed and cooled in the external recycle line and then is reintroduced into the bottom of the reactor where it passes through a distributor plate. Make-up feed streams may be added to maintain the desired reactant concentrations.
The properties of the polyolefin formed by such a process can be controlled to some extent by varying the operating conditions, including the operating temperature, co-monomer type and amount, type and quantity of catalyst, etc. Such properties include the molecular weight of the polyolefin, the molecular weight distribution of the polymer product, polymer density and the flow index of the polymer product. Mechanical properties that can be important for polyolefin include, for example, its impact resistance and environmental stress cracking resistance (ESCR), among others. In some cases, it has been found that the impact resistance and ESCR for polyolefins catalyst with chromium-based catalyst may run contrary to one another. In other words, increasing the impact resistance will result in reduced ESCR while decreasing the impact resistance will result in increased ESCR. Thus, it may be difficult to obtain a good balance between impact resistance and ESCR, particularly with chromium-based catalysts. Accordingly, improved methods of controlling polymer properties to achieve a desired balance between impact resistance and ESCR would be desirable and advantageous.